1. Field of the Invention
The present invention relates to a method of manufacturing imaging optical elements and to an optical scanning device using the imaging optical elements manufactured by the method of manufacturing imaging optical elements, and is suitable for a color image forming apparatus, in particular, a laser beam printer, a digital copying machine, or a multi-function printer.
2. Description of the Related Art
Hitherto, in an optical scanning device used in an image forming apparatus such as a laser beam printer (LBP), a light beam optically modulated and emitted from a light source unit in accordance with an image signal is periodically deflected with an optical deflector including a rotary polygon mirror (polygon mirror). Then, the deflected light beam is converged into a spot-like shape on a surface of a photosensitive recording medium (photosensitive drum) by an imaging optical system having an fθ characteristic, and the surface thereof has been optically scanned, to thereby effect image recording.
In many cases, plastic lenses, which are easy to manufacture, are used as the imaging optical elements constituting the imaging optical system used in an optical scanning device of this type. The plastic lens has a feature of being easily manufactured by injection molding. In order to achieve preferable curvature of field of light beams condensed onto the surface of a photosensitive drum, to reduce scanning line curvature on the surface of the photosensitive drum, and to achieve a preferable fθ characteristic in the optical scanning device, the shape of the optical functional surface of the plastic lens is often designed to have an aspherical shape. In such a case, by using the injection molding, a desired aspherical shape can be manufactured more easily compared to the case of using optical glass.
When the plastic lens is molded by the injection molding, it is a commonly known fact that the surface shape of the produced lens changes due to molding shrinkage of a plastic. For example, in a case where a lens is molded by using a plastic as a material, the produced lens becomes smaller than the dimensions of a cavity formed of a mirror-finish insert of a mold. In addition, the shape of the optical functional surface is also deformed by the molding shrinkage with respect to the surface shape of the mirror-finish insert. When such error does not fall within a design allowable range, the use of such lenses leads to decrease in optical performance. Examples of the optical performance changes which occur due to the molding shrinkage include focal point deviations in a main scanning direction and a sub scanning direction and an imaging position deviation (irradiation position deviation). In particular, the imaging optical element to be arranged on the side of the surface to be scanned is generally thin and long in the main scanning direction, and is thus susceptible to the effect of unevenness in temperature distribution within the mold. As a result, warp occurs in the imaging optical element. As for the warp of the imaging optical element, an improvement can be expected to some extent by taking such measures as achieving a uniform temperature distribution within the mold. However, it is difficult to completely eliminate the warp of the lens because of the construction of the mold. However, if the amount of deviation from the mold which occurs at the time of molding is stable and does not fluctuate significantly depending on the day and time of the molding and the environment, the shape of the molded product can be made within the design allowable range by correcting the error in advance by means of the mold shape.
Conventionally, there are known methods in which the mirror-finish insert is created by taking into account the shrinkage, the amount of deformation, and the like at the time of molding (Japanese Patent Application Laid-Open No. H07-060857 and Japanese Patent Application Laid-Open No. 2002-248666). The optical element molding method of Japanese Patent Application Laid-Open No. H07-060857 discloses a method in which a lens is once molded to measure the shape error of the optical functional surface thereof, and the mirror-finish insert of the mold is then corrected so as to cancel the shape error caused by the effect of uneven shrinkage of a resin. Further, the optical element manufacturing method of Japanese Patent Application Laid-Open No. 2002-248666 discloses a method in which a part of the shape of the optical functional surface is corrected so as to cancel the curvature of field based on results of measuring optical characteristics.
In a color image forming apparatus serving as an image forming apparatus, there is used an optical scanning device in which a rotary polygon mirror (deflection unit) is shared by a plurality of beams for the purpose of making the entire apparatus compact. In such an optical scanning device, scanning optical systems are arranged on both sides of the deflection unit. Further, in one of the scanning optical systems, two light beams are caused to enter one deflection surface of the deflection unit in oblique directions from above and below with respect to the sub scanning direction. Thus, in the scanning optical system on one side of the deflection unit, two surfaces to be scanned are scanned, and also in the scanning optical system on the other side, two surfaces to be scanned are scanned as well.
The scanning optical system used in such an optical scanning device is provided with an imaging optical system having an fθ characteristic for each of the surfaces to be scanned. In general, the imaging optical system is constituted by a plurality of imaging optical elements. Of the plurality of imaging optical elements constituting the imaging optical system, the imaging optical element arranged on the side of the deflection unit is shared by the two imaging optical systems, and the imaging optical elements arranged on the side of the surfaces to be scanned are used by the respective imaging optical systems. In the two imaging optical systems, the light beams are caused to enter the imaging optical elements in an oblique manner with respect to the sub scanning direction, and hence positions used in the imaging optical elements (positions which the light beams enter) (light beam passing states) vary among the respective imaging optical systems in the sub scanning direction.
In the optical element manufacturing methods disclosed in Japanese Patent Application Laid-Open No. H07-060857 and Japanese Patent Application Laid-Open No. 2002-248666, when the optical elements are manufactured by using the mold, consideration is not given to a fact that even though optical elements are arranged optically at the same position, incident positions of the light beams entering the optical elements are different. In other words, the shape error caused by the effect of the shrinkage of the resin, which differs depending on the incident position of the light beam entering the optical element, is not considered. Accordingly, it is difficult to obtain the mold that cancels the shape error of the optical functional surface for the imaging optical elements to be used in the scanning optical system in which a plurality of light beams is caused to obliquely enter the optical elements in the sub scanning cross-section so as to simultaneously scan a plurality of surfaces to be scanned.